1. Field of the Invention
This invention is related to the practice of sidetrack drilling for hydrocarbons. More specifically, this invention pertains to a system for creating a window within a vertical wellbore casing and then drilling a sidetrack wellbore through that window in a single trip.
2. Background of the Related Art
In recent years, technology has been developed which allows an operator to drill a primary vertical well, and then continue drilling an angled lateral borehole off of that vertical well at a chosen depth. Generally, the vertical wellbore is first cased with a string of casing and cemented. Then a tool known as a whipstock is positioned in the casing at the depth where deflection is desired. The whipstock is specially configured to divert milling bits and then a drill bit in a desired direction for forming a lateral borehole. This process is sometimes referred to as sidetrack, or directional, drilling.
To create a lateral wellbore, an anchor, slip mechanism, or an anchor-packer is first set in a wellbore at a desired location. This device acts as an anchor against which tools above it may be urged to activate different tool functions. The device typically has a key or other orientation indicating member. The device""s orientation is checked by running a tool such as a gyroscope indicator or measuring-while-drilling device into the wellbore.
A whipstock is next run into the wellbore. A stinger is located at the bottom of the whipstock which engages the anchor device or packer. In this respect, splined connections between the stinger and the anchor facilitate correct stinger orientation. The stinger allows the concave face of the whipstock to be properly oriented so as to direct the milling operation.
For sidetracking operations, it is most commonly known to employ a mill having cutting blades, with the mill being placed at the end of the drill pipe or other tubular column. A starting mill is releasably secured at the top of the whipstock, e.g. with a shearable setting stud connected to a pilot lug on the whipstock. Rotation of the string with the starting mill rotates the mill, causing the connection with the whipstock to be sheared.
The starting mill has a tapered portion which is slowly lowered to contact the pilot lug on the concave face of the whipstock. The starting mill moves downwardly while contacting the pilot lug or the concave portion. This urges the starting mill into contact with the casing. The casing is milled as the pilot lug is milled off. Milling of the casing is achieved by rotating the tool against the inner wall of the casing while at the same time exerting a downward force on the drill string against the whipstock. The starting mill cuts an initial window in the casing. The starting mill is then removed from the wellbore.
A window mill, e.g. on a flexible joint of drill pipe, is next lowered into the wellbore. The window mill is rotated to mill down from the initial window formed by the starting mill. A window is thereby created in the form of an elongated opening pocket. The window mill is then removed from the wellbore.
As a next step, the drill string is tripped. A drill bit is then run on drill string which is deflected by the whipstock through the freshly milled window. The drill bit engages the formation so as to directionally form the lateral borehole adjacent the window.
As can be seen, sidetracking operations which employ a whipstock and mill require the use of various tools in a certain sequence. This sequence of operation requires a plurality of xe2x80x9ctripsxe2x80x9d into the wellbore. For example, the first trip occurs after drilling has reached a depth below the desired depth for the window. The drill string is pulled so that the drill bit may be replaced with a packer. The packer is then run to a desired depth and set.
After setting the packer, the drill string is tripped to run the whipstock. The whipstock is set down hole above the packer. Technology has been recently developed which allows the milling device to be run with the whipstock. U.S. Pat. No. 6,112,812 discloses a mill which is releasably secured at the top of the whipstock, e.g. with a shearable setting stud connected to a pilot lug on the whipstock. The mill and whipstock can then be lowered into the wellbore together. Rotation of the string rotates the mill, and causes shearing of the connection with the whipstock. However, as also noted, it is necessary to start the milling process with a smaller gauge mill, and then move to progressively larger gauge mills to complete the window. In some instances, full gauge mills are not run to mill a full gauged window through the casing on a singular trip, but rather are run on subsequent trips after a starting mill is run. This requires still further trips.
Once the window is milled, the final milling device must be removed from the wellbore. At the surface, the mill is replaced with the drill bit, and drilling through the new window downhole commences.
The process of running drill string in and out of the hole is time consuming. As can be seen, multiple trips are typically required in order to complete a sidetrack drilling operation. Rig time is expensive and multiple trips take time and add to the risk that problems will occur. In an effort to reduce the number of trips, a milling device incorporating more than one mill gauge has recently been developed. Similarly, U.S. Pat. No. 6,116,336 discloses a packer and a whipstock being run together. More impressively, U.S. Pat. No. 6,112,812 discloses a milling device having both a whipstock and an anchor attached such that these three devices can b e run and operated in a single trip. However, no method has been disclosed which would combine, into a single trip, the placement of an anchor, a whipstock, a milling device, and a drill bit for finally drilling the lateral wellbore. Thus, a need exists for such a system.
In addition, the standard method for creating a casing window for a lateral hole requires the use of drilling fluids which are pumped into the formation to circulate casing cuttings, or cutting swarf, and to cool the cutting blades. This further adds to the expense of the sidetracking process. U.S. Pat. No. 5,791,417 discloses a system for opening a window in casing for sidetrack drilling operations by the use of an explosive charge. This system allows the charge to be applied to a portion of casing in the same trip as running the whipstock. However, a need still exists which would allow subsequent drilling of a lateral wellbore through the window in that same trip. Thus, a need exists for an effective xe2x80x9csingle tripxe2x80x9d method for forming a window in wellbore casing whereby a window is formed and the lateral wellbore is drilled in a single trip.
Therefore, one of the many objects of this invention is to eliminate the need for multiple trips in connection with sidetrack drilling.
Further, it is an object of the present invention to provide a system for forming a casing window for sidetrack drilling operations whereby an anchor, a whipstock, a milling device and a drill bit for drilling the lateral wellbore itself are run in the same trip.
Still further, it is an object of one embodiment of the present invention to provide a single trip system for forming a casing window for sidetrack drilling operations without rotation of the drill string.
The present invention discloses and claims a system for forming an opening, or a window, in a downhole tubular for the subsequent formation of a lateral wellbore. More specifically, a system for creating a window in a wellbore, and then drilling a sidetrack wellbore through that window, is provided. According to the system of the present invention, a series of tools is run on a drill string into the primary wellbore. These tools allow for the milling of a window in the casing of a wellbore, and then for the drilling of a lateral wellbore through that window, in the same trip.
To effectuate the system of the present invention, a drill bit is run into the primary wellbore on the lower end of a drill string. A diverter, known in the industry as a whipstock, is attached temporarily to the drill bit with a mechanically shearable connection. The whipstock is run into the wellbore along with and below the drill bit. The whipstock includes a concave face for properly diverting the drill bit into the lateral wellbore. It may also include a pilot lug for temporarily connecting the drill bit with the whipstock.
At the base of the whipstock is an anchor. The anchor is used to set the whipstock in place for sidetrack drilling operations.
A milling device is next provided. The milling device creates a window in the casing through which sidetrack drilling operations enter. In the preferred embodiment, the milling device is lowered on the drill string below the anchor. The milling device is appropriately located downhole and oriented. The milling device is then activated to create a hole through which drilling of the formation adjacent the primary wellbore is possible.
Various milling devices may be employed in connection with the system of the present invention. In one embodiment, the milling device utilizes pyrotechnic means for cutting a window through the casing. Such pyrotechnic means may include a container having an exothermic material. The exothermic material is lowered into the wellbore at a predetermined depth. Thereafter, the exothermic material is ignited and a portion of the casing therearound is destroyed, leaving a window in the casing.
In another embodiment, the milling device is a reciprocating mill in the form of a broach. The broach includes teeth for mechanically cutting an opening through the casing.
In still another embodiment, the milling device is an explosive charge. The charge is used to explosively form an opening in the casing. The explosive charge is properly designed to form a hole of desired configuration in the casing without damaging the anchor or whipstock.
In operation downhole, the milling device is employed in the system of the present invention before the anchor is set. The device""s orientation is checked by running a tool such as a gyroscope indicator or measuring-while-drilling device into the wellbore. The milling process is then conducted. When milling is completed, the whipstock is lowered into the wellbore and located adjacent the newly formed window. Once the whipstock is in place and properly oriented, the anchor is set. Setting the anchoring device allows the drill bit to act against the whipstock and to be diverted through the window and into the formation in order to drill a lateral wellbore.
In the preferred embodiment, the drill bit is a fixed drill bit. The drill bit is temporarily attached to the whipstock by a pilot lug. The pilot lug is releasably connected to the drill bit at its upper end by shearable setting studs, and connected at its lower end to the whipstock. Pulling on the set drill string shears the setting studs, freeing the drill bit from the whipstock. Slips extend from the stinger and engage the side of the wellbore to prevent movement of the whipstock in the wellbore; and locking apparatus locks the stinger in a packer when a packer is used. Rotation of the drill string rotates the drill bit. The drill bit is slowly lowered to contact the pilot lug on the concave face of the whipstock. This forces the drill bit through the formed window, and a new lateral wellbore is drilled as the pilot lug is milled off.
In yet another embodiment, an apparatus in run into a wellbore, the apparatus including a run in string of tubulars, a drill bit, a diverting device and a milling device. Thereafter, an aperture is formed in a casing wall with the milling device and the milling device is then disconnected from the drill bit and string. Thereafter, the bit is directed through the newly formed aperture in the casing and a lateral wellbore is formed. The diverting device can include a whipstock, a rotary steering means or a bent sub disclosed proximate the drill bit.